输入重定向不起作用？

Question

My instructions: Write a program that repeatedly (until end-of-file) reads in a character from the input stream. If the character is upper case, change it to lower case and write it to the output stream. For all other characters, write the character unchanged to the output stream. Use getchar() for input, Use putchar() for output, and use input redirection for connecting the input file to the program

My project name is Input and my textfile is input.txt. When I run it I type "Input < input.txt" The program just mimics that on the command window though so how do I get it to read from the text file?

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>

int main()
{
  char c, x;


   c=getchar();

  while (c != EOF)
 {

    c=getchar();
     x = tolower(c);
     if (isupper(c))
    {
        putchar(x);

    }
    else 
    {
        putchar(c);

    }

 }
    system("Pause");
}

Answer 1

I believe the problem is that you do not want to go to the program and type in Input < input.txt . Instead, you want to open a terminal program, change directory into the directory containing the project, and then run the program from the command line by writing Input < input.txt . This starts up the program and uses the contents of input.txt as the standard input stream, rather than reading text from the console.

That said, there are two bugs in your code. First, note that you have the line

c = getchar();

outside of your loop that does the input reading. You then immediately call

c = getchar();

again inside the loop. This means that you are discarding the very first character that you read in.

Second, your loop runs one more time than it needs to. If the second call to getchar() returns EOF , you do not detect this until after the current loop iteration finishes. This is because your check for EOF is at the top of the loop, which isn't reached until after you've already printed out the EOF character. To fix this, consider using the loop-and-a-half idiom and breaking out of the loop in the middle:

for (;;) {
    /* Read data. */
    if (/* no data left */) break;

    /* Process data. */
}

Stylistically, are you sure that you need both the if and else branches here? Recall that tolower will not change the values of characters that aren't upper-case, so having one case for upper-case letters and one for lower-case is redundant. You can just output the character produced by tolower without the special cases.

Hope this helps!

Answer 2

The idiomatic way of doing this for ASCII text only is:

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>


int main()
{
    int c;

    while((c = getchar()) != EOF)
        putchar(tolower(c));

    exit(EXIT_SUCCESS);
}

As textbooks are so fond of saying, UTF-8 is left as an easy exercise to the reader ;-)